Get the towels ready. Stock up on those smelling salts. And water, buckets of water. Whatever you do, prepare yourself – because over the next 23 days, the political pundits are going to give you a battering.

Flick on the radio and a platoon of academics will burst forth, extrapolating from the latest polls. Open the newspaper, and face a shelling by the heavy-lead certainties of commentators and politicians. Settle back in front of the telly, and a battery of experts will pummel you with predictions. Gordon Brown will be deemed be a goner. Unless, that is, the tide turns against David Cameron. The battle will be won in the Midlands. Until Kent is in the limelight. All this will be said in short, declarative sentences. Like these. For extra authority.

Apart from being exhausting, this high-intensity forecasting is largely worthless. Cast your mind back to June last year, when yet another Labour putsch was being launched against the prime minister. Trawling through the comment pages published in the week when the coup was at its height – with ministers resigning, and local and European elections looming – I found 20 columns and leading articles in the Times, Telegraph, Independent and Guardian discussing whether Brown would survive. Of those, half predicted he would go, while only a quarter thought he might stay (the rest, perhaps wisely, didn't chance their arm). If the broadsheet fortune-tellers could not assess the outcome of a backroom plot featuring a few ministers and MPs, how far should we trust their judgment on what tens of millions of voters will do?

This is probably unfair, and certainly unscientific. Brown did come close to being toppled last summer, and cabinet conspirators tend to duck out of ICM-style opinion polls. But, on the broad question of whether we should set much store by political predictions, the answer is a flat no.

Just ask Phil Tetlock. A psychologist at Berkeley, he spent two decades asking experts of all stripes and ideological hues – from professors to journalists, Marxists to free-marketeers – to make precise forecasts both within their specialist fields and far outside. He asked them everything from where the Dow Jones would go to whether the Québécois would secede from Canada, amassing a database of 82,361 predictions from 284 experts. Finally, Tetlock compared the forecasts to what actually happened.

The results looked career-threateningly bad. The predictions of the individual experts were only slightly more accurate than guesses made at random – only a bit better, that is, than if a chimp had chucked darts at a board. The experts performed worse than basic computer algorithms. And Tetlock found something else to give Newsnight aficionados pause: those prognosticators with the biggest media profiles were especially bad.

He put his research into a gem of a book called Expert Political Judgement – but woeful forecasters don't dwell solely in Washington or Westminster. Studies show they are also common in psychology, sports, stock markets – so prevalent, in fact, that in 1980 Scott Armstrong, an academic at Wharton business school in the US, came up with the seer-sucker theory. It ran: "No matter how much evidence exists that seers do not exist, suckers will pay for the existence of seers."

The field with the greatest proliferation of seers must be economics, whose blithe confidence contributed to the crash of 2008, and to the disgrace of an entire profession. Last weekend, some of the best-known economists gathered in Cambridge for a conference organised by George Soros to discuss the crisis in their discipline. "Economists didn't see any of this coming," Nobel Laureate Joseph Stiglitz told me. "All their models were too simplistic – and based on the premise that markets worked." And his remedy? "Economics needs to become a lot less theological."

Which is excellent advice for political pundits, too. There is obviously a role for experts – just not as oracles (although Tetlock did note that the group who performed worst of all were undergraduates with no specialism). But being able to analyse what people do, putting it into interesting context, and modestly suggesting how things might be done better, are all valuable skills.

Admittedly, none of this would cut the mustard with Harry S Truman, the US president who asked for a one-armed economist because he was so tired of answers that ran on-the-one-hand-and-on-the-other. And clear-cut answers certainly go down better on television, where qualifications are best left to the Open University.

Having strident views, however wrong, is probably also better for an expert's career. After Tetlock presented his findings in San Francisco a few years ago, his host, Stewart Brand, told a story that officials haven't since denied.

In the 1980s, Brand's business partner was asked to speak to the top brass at the CIA about the outlook for the Soviet Union. He ran through a few scenarios, including one in which the Soviet bloc broke up. One sign that this might happen, he said, would be if a relative- unknown called Mikhail Gorbachev rose up through the party ranks.

At this point, one of the CIA analysts jumped in. The presentation was fine, he said, but there was no way the Soviet Union was going to break up – not in his lifetime, not in his children's lifetime.

That analyst's name, said Brand, was Robert M Gates. He is now the US secretary of defence.